27.1 Where does crude oil come from?.
The generally-accepted origin of crude oil is from plant life up to 3
billion years ago, but predominantly from 100 to 600 million years ago [1].
"Dead vegetarian dino dinner" is more correct than "dead dinos".
The molecular structure of the hydrocarbons and other compounds present
in fossil fuels can be linked to the leaf waxes and other plant molecules of
marine and terrestrial plants believed to exist during that era. There are
various biogenic marker chemicals such as isoprenoids from terpenes,
porphyrins and aromatics from natural pigments, pristane and phytane from
the hydrolysis of chlorophyll, and normal alkanes from waxes, whose size
and shape can not be explained by known geological processes [2]. The
presence of optical activity and the carbon isotopic ratios also indicate a
biological origin [3]. There is another hypothesis that suggests crude oil
is derived from methane from the earth's interior. The current main
proponent of this abiotic theory is Thomas Gold, however abiotic and
extraterrestrial origins for fossil fuels were also considered at the turn
of the century, and were discarded then. A large amount of additional
evidence for the biological origin of crude oil has accumulated, however
Professor Gold still actively promotes his theory worldwide, even though
it does not account for the location and composition of all crude oils.
27.2 What are CNG/LPG/gasoline/kerosine/diesel?.
Crude oil consists mainly of hydrocarbons with carbon numbers between one and
forty. The petroleum refinery takes this product and refines it into several
fuel fractions that are optimised for their intended application. For spark
ignition engines, the very volatile and branched chain alkane hydrocarbons
have desirable combustion properties, and several fractions are produced.
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20%
ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to
pentane. The major disadvantage of compressed gaseous fuels is the reduced
range. Vehicles may have between one to three cylinders ( 25 MPa, 90-120
litre capacity), and they usually provide about 50% of the gasoline range.
LPG ( Liquefied Petroleum Gas ) is predominantly propane with iso-butane
and n-butane. It has one major advantage over CNG, the tanks do not have
to be high pressure, and the fuel is stored as a liquid. The fuel offers
most of the environmental benefits of CNG, including high octane - which
means higher compression, more efficient, engines can be used. Approximately
20-25% more fuel than gasoline is required, unless the engine is optimised
( CR 12:1 ) for LPG, in which case there is no decrease in power or any
significant increase in fuel consumption [4,5].
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. A detailed description of the
composition of gasoline, along with the properties and compositions of CNG,
LPG, and oxygenates can be found in the Gasoline FAQ, which is posted monthly
to rec.autos.tech.
Kerosine is a hydrocarbon fraction that typically distils between 170-270C
(narrow cut kerosine, or Jet A1) or 100-250C ( wide cut kerosine, or JP-4 ).
It contains around 20% of aromatics, however the aromatic content will be
reduced for high quality lighting kerosines, as the aromatics reduce the
smoke point. The major use for kerosines today is as aviation turbine (jet)
fuels. Special properties are required for that application, including high
flash point for safe refuelling ( 38C for Jet A1 ), low freezing point for
high altitude flying ( -47C for Jet A1 ), and good water separation
characteristics. Details can be found in any petroleum refining text and
Kirk Othmer.
Diesel is used in compression ignition engines, and is a hydrocarbon fraction
that typically distils between 250-380C. Diesel engines use the Cetane
(n-hexadecane) rating to assess ignition delay. Normal alkanes have a high
cetane rating, ( nC16=100 ) whereas aromatics ( alpha methylnaphthalene = 0 )
and iso-alkanes ( 2,2,4,4,6,8,8-hexamethylnonane = 15 ) have low ratings,
which represent long ignition delays. Because of the size of the hydrocarbons,
the low temperature flow properties control the composition of diesel, and
additives are used to prevent filter blocking in cooler temperatures. There
are usually summer and winter grades. Environmental legislation is reducing
the amount of aromatics and sulfur permitted in diesel, and the emission of
small particulates ( diameters of <10um ) that are considered possibly
carcinogenic, and are known to cause other adverse health effects. Details
can be found in any petroleum refining text and Kirk Othmer.
27.3 What are oxygenates?.
Oxygenates are just pre-used hydrocarbons :-). They contain oxygen, which can
not provide energy, but their structure provides a reasonable anti-knock
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases [6]. Most oxygenates used
in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain
1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and
MTBE was first used in commercial gasolines in Italy in 1973, and was first
used in the US by ARCO in 1979. The relative advantages of aromatics and
oxygenates as environmentally-friendly and low toxicity octane-enhancers are
still being researched.
Ethanol C-C-O-H C2H5OH
C
|
Methyl tertiary butyl ether C-C-O-C C4H9OCH3
(aka tertiary butyl methyl ether ) |
C
They can be produced from fossil fuels eg methanol (MeOH), methyl tertiary
butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg
ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). MTBE is produced by
reacting methanol ( from natural gas ) with isobutylene in the liquid phase
over an acidic ion-exchange resin catalyst at 100C. The isobutylene was
initially from refinery catalytic crackers or petrochemical olefin plants,
but these days larger plants produce it from butanes.
Oxygenates have significantly different physical properties to hydrocarbons,
and the levels that can be added to gasolines are controlled by the EPA in
the US, with waivers being granted for some combinations. Initially the
oxygenates were added to hydrocarbon fractions that were slightly-modified
unleaded gasoline fractions, and these were commonly known as "oxygenated"
gasolines. In 1995, the hydrocarbon fraction was significantly modified, and
these gasolines are called "reformulated gasolines" ( RFGs ). The change to
reformulated gasoline requires oxygenates to provide octane, but also that
the hydrocarbon composition of RFG must be significantly more modified than
the existing oxygenated gasolines to reduce unsaturates, volatility, benzene,
and the reactivity of emissions.
Oxygenates that are added to gasoline function in two ways. Firstly they
have high blending octane, and so can replace high octane aromatics
in the fuel. These aromatics are responsible for disproportionate amounts
of CO and HC exhaust emissions. This is called the "aromatic substitution
effect". Oxygenates also cause engines without sophisticated engine
management systems to move to the lean side of stoichiometry, thus reducing
emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can
reduce HC by 10%)[7]. However, on vehicles with engine management systems,
the fuel volume will be increased to bring the stoichiometry back to
the preferred optimum setting. Oxygen in the fuel can not contribute
energy, consequently the fuel has less energy content. For the same
efficiency and power output, more fuel has to be burnt, and the slight
improvements in combustion efficiency that oxygenates provide on some
engines usually do not completely compensate for the oxygen.
There are huge number of chemical mechanisms involved in the pre-flame
reactions of gasoline combustion. Although both alkyl leads and oxygenates
are effective at suppressing knock, the chemical modes through which they
act are entirely different. MTBE works by retarding the progress of the low
temperature or cool-flame reactions, consuming radical species, particularly
OH radicals and producing isobutene. The isobutene in turn consumes
additional OH radicals and produces unreactive, resonantly stabilised
radicals such as allyl and methyl allyl, as well as stable species such as
allene, which resist further oxidation [8,9].
The major concern with oxygenates is no longer that they may not be
effective at reducing atmospheric pollution, but that their greater water
solubility, and very slow biodegradability, can result in groundwater
pollution that may be difficult to remove. Their toxicological and
environmental effects are also still being researched.
27.4 What is petroleum ether?.
Petroleum ether ( aka petroleum spirits ) is a narrow alkane hydrocarbon
distillate fraction from crude oil. The names "ether" and "spirit" refer
to the very volatile nature of the solvent, and petroleum ether does not
have the ether ( Cx-O-Cy ) linkage, but solely consists of hydrocarbons.
Petroleum ethers are defined by their boiling range, and that is typically
20C. Typical fractions are 20-40C, 40-60C, 60-80C, 80-100C, 100-120C etc.
up to 200C. There are specially refined grades that have any aromatic
hydrocarbons removed, and there are specially named grades, eg pentane
fraction (30-40C), hexane fraction (60-80C, 67-70C). It is important to
note that most "hexane" fractions are mixtures of hydrocarbons, and pure
normal hexane is usually described as "n-hexane".
27.5 What is naphtha?.
Naphtha is a refined light distillate fraction, usually boiling below 250C,
but often with a fairly wide boiling range. Gasoline and kerosine are the
most well-known, but there are a whole range of special-purpose hydrocarbon
fractions that can be described as naphtha. The petroleum refining industry
calls the 0-100C fraction from the distillation of crude oil "light virgin
naphtha" and the 100-200C fraction " heavy virgin naphtha". The product
stream from the fluid catalytic cracker is often split into three fractions,
<105C = "light FCC naphtha", 105-160C = "intermediate FCC naphtha" and
160-200C "heavy FCC naphtha".
27.6 What are white spirits?.
White spirits are petroleum fractions that boil between 150-220C. They can
have aromatics contents between 0-100%, and Shell lists eight grades with
aromatics contents below 50%, and six grades with aromatics contents above
50%. The two common "white spirits" are defined by British Standard 245,
which states Type A should have aromatics content of less that 25% v/v and
Type B should have an aromatics content of 25-50% v/v. The most common
" white spirit" is type A, and it typically has an aromatics content of
20%, boils between 150-200C, and has an aniline point of 58C, and is
sometimes known as Low Aromatic White Spirits. The next most common is
Mineral Turpentine (aka High Aromatic White Spirits ), which typically has
an aromatics content of 50%, boils between 150-200C and has an aniline
point of 25C. For safety reasons, most White Spirits have Flash Points
above ambient, and usually above 35C. Note that "white gas" is not white
spirits, but is a volatile gasoline fraction that has a flash point below
0C, which is also known by several other names. Do not confuse the two
when purchasing fuel for camping stoves and lamps, ensure you purchase the
correct fuel.
27.7 What are biofuels?.
Biofuels are produced from biomass ( land and aquatic vegetation, animal
wastes, and photosynthetic organisms ), and are thus considered renewable
within relatively short time-frames. Examples of biofuels include wood,
dried animal dung, methyl esters from triglyceride oils, and methane from
land-fills. The renewable aspect of most biofuels is essentially the use
of solar energy to grow crops that can be converted to energy. There is
a large monograph "Fuels from Biomass" in Kirk Othmer, and the subject
is frequently discussed in alt.energy.renewable, sci.energy, and
sci.energy.hydrogen.
27.8 How can I convert cooking oil into diesel fuel?.
Diesel engines can run on plant and animal triglycerides such as tallow
and seed oils, however most trials have resulted in reduced engine life, or
increased service costs. The solution is to transesterify the triglycerides
into esters, taking care to avoid the formation of monoacylglycerides
that will precipitate out at low temperatures or when diesel is encountered.
There are several plants in Austria that produce Rapeseed Oil Methyl Esters
as fuels for diesel engines. The economics of the process are very
dependant on the price of diesel and the market for the glycerol byproduct.
The common catalysts used to transesterify triglycerides are sodium
hydroxide, sodium methoxide and potassium carbonate. If the esters are to
be blended with diesel fuel, then a two stage reaction is usually required
to ensure that monoacylglycerides are kept below 0.05%. Usually this
involves using 22g of methanol ( containing 0.6g of sodium hydroxide ) and
100g of tallow refluxed for 30 minutes. The mixture is cooled, the glycerol
layer removed, and a further 0.2g of sodium hydroxide is reacted for 5
minutes at 35C in a stirred reactor. The glycerol phase is allowed to
separate, and the ester phase is washed with water to remove residual
catalyst, glycerol and methanol. Note that sodium hydroxide is the most
cost-effective catalyst, but also has the worst tendency to form soaps.
The catalyst and methanol can be of industrial grade without further
purification required, however care should be taken to prevent additional
water entering the reaction [10].
The fuel can be converted into other esters, such as ethyl and butyl, but
it really depends on the availability of cheap alcohol along with the
desired properties of the fuels. The effect of catalysts, reagent ratio,
temperature, and moisture on the production of methyl, ethyl, and butyl
esters from some common oils has been investigated [11]. The New Zealand
government investigated a wide range of techniques for turning various
vegetable and animal triglycerides into esters for diesel, and the reports
cover many aspects of the kinetics and efficiencies [12]. There is a general
overview of the current processes and technology available in Inform [13].
A specific technique for analysing the monoglycerides has been published
[14], however I have found that acetylation followed by narrow bore
( 0.1mm ID ) capillary chromatography is faster and cheaper.

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